Do you mean to surround all of crc32 functions with ifdef _KERNEL?
crc32.c doesn't have _KERNEL.
It looks easy to compile with sys/crc32.h header but there is no library created for user land usage and also some architectures have assembly code as well. I don't think linking with kernel crc32 is easy.

After research, I found that our crc32 functions are based on the code written
by Gary S Brown. We also have many other *crc32.[hc] files, too. Rather than,
prefixing with kern, prefix with his initial to honor his work.
It looks kern prefix is used for sub-systems.